In a design of GaAs devices it is important to provide low resistant ohmic contact, that has a uniform interface which contacts the GaAs substrate. Surface protrusions or dendrites are undesirable. In one conventional system, Ni, Au-Ge, Ni and Au layers are sequentially deposited on the surface of gallium arsenide. This system has been extensively used, however, it suffers from a number of problems. The reaction of this system at the interface with the gallium arsenide is non-uniform and produces dendrites which can deeply penetrate into the gallium arsenide substrate and seriously limits the device performance. Another disadvantage is that the contact resistivity is non-reproducible. The non-uniformity is attributed to the reaction involving some liquid phases and the presence of interfacial oxides. When devices are scaled down to submicron dimensions, non-uniformity of the interface microstructure could seriously limit the device performance.
In U.S. Pat. No. 4,301,188 to Niehaus another process for producing contacts to gallium arsenide substrates is shown. In this arrangement a gold germanium alloy is directly deposited on a gallium arsenide surface. Onto the gold germanium layer there is deposited a silver layer and onto this silver layer is deposited a gold layer. This arrangement significantly reduces dendrites, but as shown in the comparative example later in the Detailed Description of the Invention, it still suffers from some problems.